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2 Answers
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The error message is somewhat strange as presented, but when you get this message creating a virtual column, the value for maximum is the minimum length that you need to specify to be able to create the column.

The reason for this is that Oracle doesn't know the maximum length of the expression to_char(a) and assumes that it will be 40 characters: 38 places of precision for the number, plus 1 for each of the decimal point and positive/negative indicator.

Add 2 characters for "A:" and the minimum length that Oracle judges as "safe" is 42.